Methods which employ countercurrent liquid-solids extraction systems to convey the subdivided solids to be extracted in one direction through a tank, and flow the liquid, usually water, in the opposite direction through the subdivided solids are well known. During the contact of the liquid and solids as they move in opposite directions, soluble substances are extracted from the solids, as by osmotic and lixiviation transfer, and are removed with the enriched extraction liquid.
Apparatus employed for performing countercurrent extraction are of several different configurations. They may be in the form of a Robert battery, a set of identical cells which are filled and emptied alternatively in rotation. They may be in the form of slope diffusers as illustrated by U.S. Pat. No. 2,885,311 Bruniche-Olsen et al. and Silver U.S. Pat. No. 3,573,892; vertical tower diffusers, as illustrated by Schaffer et al. U.S. Pat. No. 3,142,589; and horizontal diffusers, for example, the R. T. diffuser.
In the case of vertical and horizontal diffusers, the diffusers are preceded by a hydrator that prepares the animal or plant materials for extraction by treating with hot water. The hot water "kills" the cells, so that cell membranes permit osmotic transfer of soluble substances through the cell walls by the extraction liquid.
The hydrators and continuous diffusers use screens at various locations for controlling the subdivided solids. The screens may convey or push the solids through the diffuser while allowing the liquid to pass therethrough. Because of the hydraulic pressures exerted against the screens, especially when they have a mat or matrix of solids, and particularly because of their need to convey solids, as in the case of horizontal and slope diffusers, the screens must maintain structural integrity during use, and for that reason have been made of plates.
In the past, small particles and fines contained in the larger subdivided solids were a problem. Ordinarily, the smallest openings that have been drilled or cast in such plates are 1/8 inch in diameter, a size sufficiently large to pass the small particles and fines. For example, screens for separating the liquid from solids in such apparatus are ordinarily plates that are about 1/8 to 1/4 inch thick ordinarily containing drilled or punched holes about 9/32 inch across, a size that easily passes small particles and fines so that the screens cannot effectively control small particles and fines. Likewise, the scroll flights for conveying solids from one end to the other in vertical, horizontal, or slope diffusers are made of plates about 1/4 to 1/2 inch thick which may contain holes about 1/4 inch to 1/2 inch across; again, holes of such size will easily pass small particles and fines so that the latter cannot be effectively controlled. In the tower diffuser, scroll flights generally have no holes at all but just narrow sections of scroll flights. Scroll flights with or without holes, heretofore, have been ineffective in controlling large amounts of small particles and fines.
The holes or slots in the separation screen have had to be large enough not to plug with subdivided solids. Therefore the depth of the holes has had to be somewhat less than the diameter of the holes or width of the slots, so that particles nearly the same diameter or width would not be caught in the hole or slot, thereby blocking the flow of extracting liquid through the separation screen. Furthermore, as in the case of continuous diffusers extracting sugar from sliced sugar beets called cossettes, the holes had to be larger than the cross sectional area of the cossette so that a cossette starting through the hole lengthwise would not "hang up" in the hole thereby blocking that hole to the passage of extracting liquid. Therefore holes in the diffuser separation screens were customarily larger than 1/4 of an inch in diameter. Slots sometimes were made narrower than 1/4 of an inch but were often subject to plugging as a cossette would lie along the length of the slot and block the passage of extracting liquid.
Continuous diffusers in the past, operating on extracting sucrose from sugar beets, have used screen wipers only on the separation screen between the juice compartment and the active extraction sections of the continuous diffusers: slope, horizontal, and tower (batch diffusers historically have had no separation screens). Screen wipers have been made of brass or micarta materials with a pointed edge designed to cut off and lift up cossettes that have partially entered the holes of the screen (9/32s of an inch or larger) or slots of the screen (1/8 of an inch or larger). Given the propensity for foreign materials to catch in these holes, the clean-cutting pointed edge of the screen wipers has been often chipped off, worn off, or rounded so that cossettes have been mashed into the holes, causing further blockage to the extracting liquid.
Therefore, in the past, continuous diffusers, and even batch diffusers, have been ineffective for processing materials containing small particles and fines. In particular, the scroll conveyors in vertical and slope diffusers have been largely ineffective in pushing the small particles and fines countercurrent to the water in the countercurrent system. The ineffectiveness in controlling small particles and fines has been off-set, in part, by the formation in some cases of a mat or matrix of fibers and large particles that capture the small particles and fines, so that, to some extent, even though the small particles and fines can pass through the holes of the separation screens and scroll flights, nevertheless they can be captured and conveyed to some degree, provided the fines are not present in too great an amount. For this reason, the preparation of materials to be extracted has attempted to keep the small particles and fines to a minimum, for example, usually to less than about 5% by weight, and desirably to less than about 3% by weight, of the solids to be treated.
The inability to prevent small particles and fines from being carried with the extracting liquid has limited the use of batch and continuous diffusers. Although batch and continuous diffusers have found wide usage for extraction in the beet sugar and cane sugar industries, the application of extraction to many other materials has been limited, because such other materials produce large amounts of small particles and fines that conventional batch and continuous diffusers do not adequately control. Since many solid substances, when subdivided, create large amounts of small particles and fines that cannot be controlled by conventional hydrators and diffusers, the application of batch or continuous diffusers to products outside of the sugar beet and sugar cane industries has been limited. For example, in preparing madder root for extraction by pulverizing, in one case, about 10% by weight of the pulverized root was small particles and fines. Many other products, when subdivided, produce small particles, fines, and powder that cannot be handled by conventional diffusers.